Annealing strip during cold rolling

ABSTRACT

A METHOD AND APPARATUS FOR CONTINUOUSLY ANNEALING COLD ROLLED SHEET STRIP IN WHICH A TEMPERATURE SUFFICIENT TO PROVIDE A RECRYSTALLIZED AND ANNEALED STRUCTURE IS ATTAINED BY UTILIZING THE ENERGY OF DEFORMATION TO PROVIDE THE REQUIRED ULTRA-FAST HEAT-UP RATE.

Oct. 19, 1971 w. L. ROBERTS ANNEALING STRIP DURING COLD ROLLING Filed Jan. 29, 1970 3 Sheets-Sheet l (PRIOR ART) (rH/s INVENTION) sLAas SL485 HOT STRIP ,uor STRIP MILL (a) MILL (a) PICKLE LINE PYICKLE LINE PRIMARY cm 0 SPEC/A L L r 05- s/azvso coLo REDUCTION MILL (c) ROLL/Nam TINN/NG LINE (f) J ELECTROLYTIC CLEANING LINE (d) l"'"' I CONTINUOUS |ANNEALIN6 LINE/e) L J I 80X ANNEALI/VG (0/ I v T EL 50 TROL Y r/c EL 50 TROLY r/c T/N PLATE -7'/N PLATE INVENTOR. WILL 1AM L. ROBER T5 mg M A! funny Oct. 19, 1971 w, ROBERTS 3,513,425

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' Attorney United States Patent "ice 3,613,425 ANNEALING STRIP DURING COLD ROLLING William L. Roberts, Franklin Township, Westmoreland County, Pa., assignor to United States Steel Corporation Filed Jan. 29, 1970, Ser. No. 6,861 Int. Cl. B21b 37/00, 37/04, 1/00; B21d 31/00 US. Cl. 72-202 14 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for continuously annealing cold rolled sheet strip in which a temperature sufiicient to provide a recrystallized and annealed structure is attained by utilizing the energy of deformation to provide the required ultra-fast heat-up rate.

This invention relates to a method and apparatus for the continuous anneal of light-gage cold rolled steel. The method is particularly suitable for the production of sheet and tinplate stock. The novel reduction mill disclosed herein is of a simpler and more economical construction than a conventional tandem mill and is thereby easier to install and operate.

In the conventional cold reduction of pickled, hot-rolled strip to black plate and tinplate, the strip is reduced approximately 80 to 90 percent by a tandem mill of 4, 5, or 6 stands. Each stand normally employs four-high rolls to provide a pass reduction of between 25 .to 45 percent on all stands except the last, where reduction falls to between 10 and 30 percent. Heavy reduction at high speed generates a considerable amount of heat which not only raises the temperature of the product but also that of the rolls. This heat is normally dissipated by a system of flood lubrication in which jets of coolant oil are directed against the roll bodies and the surface of the steel. The resultant steel temperature generally runs between 150 and 250 F.; however, temperatures higher than 400 F. are reached (i.e., increases of about 350 F. are achieved) on conventional high speed tandem mills rolling tinplate stock. Because of work hardening, the strip emerging from the mill is fully hard with yield strengths of 125,000 to 150,000 p.s.i. and with little or no ductility. Therefore, the as-rolled strip is unsuitable for conventional can manufacture and must be cleaned to remove the rolling lubricant and an nealed to restore its ductility. The strip is therefore box annealed or as in the more modern processes, continuous annealed in a non-oxidizing atmosphere to provide lengths of material, thousands of feet long. In this latter treatment, the cold reduced strand travels at high speed through a heating zone having a controlled atmosphere, where it is brought to a temperature just above the lower critical temperature in a very short time (i.e. about 20 seconds), recrystallizes almost instantaneously, passes through a cooling zone and emerges into the air cold enough to avoid oxidation. The extremely short time at temperature (generally about 20-25 seconds) is effective because recrystallization has been suppressed by the rapid temperature rise; the resultant increase in energy level at potential nucleation centers causes the microstructure to flash over once recrystallization begins.

In these conventional processes heating rates of the order of 50 F. per second are obtained, generally by the use of gas-fired radiant furnaces. During the approximately 20 seconds which are required to reach recrystallization temperature at this rate, recovery processes are initiated and proceed, thereby diminishing the amount of stored energy available to drive the recrystallization. A more rapid attainment of recrystallization temperature would therefore be desirable, since the lower the length of time during which the steel is held in the recovery temperature range,

3,013,425 Patented cf. 19, 1971 the more energy would be retained in the workpiece to drive the recrystallization process. Thus the experiment of E. W. Williams (Iron & Steel Institute, Spec. Report, #79, (1963) pp. 8792) on 0.010 inch, cold rolled, low carbon steel showed that a fully recrystallized material can be obtained in 1.2 seconds with heating rates of the order of 1300 F second, followed by a water quench.

It is therefore an object of this invention to provide a process and apparatus whereby annealing and recrystallization of the strip may be attained by utilizing the energy of deformation to reach recrystallization temperatures at ultra-fast heating rates, i.e. rates much in excess of 1000 F./second.

It is a further object of this invention to significantly reduce the cost of strip processing apparatus by eliminating both the annealing facilities and the extra rolling stands required in conventional continuous annealing facilities.

It is another object to reduce the operating costs of strip rolling and annealing facilities.

Still another object is to provide an apparatus capable of controlling the degree of annealing achieved, while eliminating the usual annealing facilities.

Yet another object is to achieve a product having an annealed center and a work hardened surface provided by chilled work rolls.

These and other objects will be more apparent by referring to the following disclosure and drawings in which:

FIG. 1 is a block diagram depicting a comparison of the conventional processing of tinplate with the process of the subject invention;

FIG. 2 is an illustration of the specially designed cold rolling mill suitable for carrying out the process of this invention;

FIG. 3 is a schematic diagram of the temperature profile attairied by the strip along the pass line of the mill of FIG. 2; and

' In the more modern facilities, steps (d) and (e) are combined in a continuous annealing line (e'). The greatly simplified processing sequence of the present invention is illustrated in FIG. 1(b) wherein the numerous stands of the primary cold reduction mill is replaced by the specially designed mill and steps (d) and (e), or, alternatively, step (e'), are completely eliminated.

FIG. 2 depicts an apparatus which is particularly suitable for performing the process of this invention. A roll stand, designated generally by the numeral 1 consists of work rolls 2 and backup rolls 3. A lubricant spray system 4 is located on the entry side of stand 1, and an arrangement of water sprays 5 and baffles 6 is positioned on the exit side of the stand 1. Stand 7 comprises work rolls 8, (considerably larger than conventional work rolls) each of which is backed up by a pair of rolls 9. A lubricant spray system 10 and wipers 11 are located on the entry side of this stand. A hood 12 is positioned between an induction heater 13, and the entry side of stand 7 and is connected by piping 14 to a source of nitrogen or other inert gas (not shown). An arrangement of high pressure water sprays 15, baffles 16 and cooling fluid sprays 17 is provided on the exit side of the stand.

The practice and operation of this invention may be more fully understood by reference to both FIGS. 2 and 3. The process must be operated so that the initial temperature of the strip (T the increase in temperature due to rolling at the first stand (AT the temperature decrease due to cooling between stands (AT increase in temperature provided by the heater (AT decrease due to air or inert gas cooling (ATQ-lincrease (AT due to the main bite of roll stand 7 will be greater than the minimum temperature required T for the desired recrystallization. In practice this latter temperature will vary, depending on the nature and concentration of the alloying elements present. Work by G, K. Lvou (Russian Metallurgy, #2, 1966, pp. 513), for example, employing heating rates of 1500 C./second has shown that, independent of concentration, elements such as Mn and Cr cause an increase in recrystallization temperature, while Ni reduces same. On the other hand, Ti, Al, and V, effect first a decrease in recrystallization temperature and then an increase, with increasing concentration of the alloying element. For plain carbon and most low alloy steels, the recrystallization temperature T will vary between 1100 F. and 1400 F.

The temperatures which must be attained in the various steps are formulated below:

It will thus be seen that if the initial temperature of the strip T is sulficiently high, or if AT is sufliciently large, there will be no need to impart heat by use of the first stand (AT or by the heater (AT Under such circumstances, either one or both (first stand or heater) may be eliminated. However, while not necessary in all cases the use of at least one initial stand provides a number of advantages: (a) an increase in the overall reduction given to the strip; (b) an aid in raising the temperature of the strip through deformation; a high entry tension to the second stand (thus reducing the rolling force); and (d) reduction of the thermal energy which the heater and/or the final deformation must impart. In most cases it also will be necessary to employ a heater which is capable of imparting heat to the strip at a very rapid rate. Induction heating is simplest in this respect although other methods well known to the art may be employed. For example, electron beam heating is also capable of providing a desirable heat-up rate; however, the apparatus would be more complicated in that a vacuum chamber would be required in most instances. Conventional radiative or conventional furnaces would involve too long a pass to be within the realm of practicality.

It will be obvious to those skilled in the art that the greater the amount of deformation provided in the last state, the larger will the temperature rise (AT thereby diminishing the need for as much heat to be imparted by the initial deformation (AT or by the heater (AT The maximum deformation is only limited by practical expediences such as input power available and size of working rolls, In general, roll sizes will vary from 30 inches to 100 inches in diameter. A minimum of about 30 inches is required to provide the rolling force and still attain adequate rigidity, and to provide a large enough surface for cooling (a portion of the energy fed to the rolls must be removed as thermal energy from roll surfaces by cooling sprays or other suitable means to prevent roll distortion and lubricating oil degradation). This minimum will also provide a large enough contact surface, to prevent spalling of the roll surfaces due to excessive stresses resulting from the high forces required. The upper limit (about 100") is determined only by ability of present day roll grinders to handle such a large roll. The maximum rolling speed is likewise established by the practicality of input power available both to the heater stage (AT if employed, and the mill stands (ATM-A73). For conventional strip widths and thicknesses, the maximum speed is about 5000 feet per minute. On the other hand, rolling speed cannot be too low since 4 this will (1) cause an undesirable loss of heat from the strip (AT and AT and (2) require higher rolling loads at the final stand, due to higher friction in the roll bite.

Strip having thicknesses of about 0.05 inch to about 0.25 inch can be processed by the contemplated invention. The lower value being determined primarily by the limitation on productivity of the hot strip mill in feeding hot band of lighter gage (i.e., tonnage rates would be too low) while strip thicker than about 0.25 inch would be too stitl to handle practically. Similarly, the continuously rolled and annealed product strip would not be less than about 0.006 inch gage due to low tonnage rates which could be obtained, and the resulting greater heat loss from the strip.

In the apparatus of FIG. 2, the process operates as follows. From the pickling line, the strip 5 is fed through the work rolls 2 of stand 1, where the temperature is increased by AT Since no coolant is applied directly to the strip as it leaves this stand, it is only slightly air-cooled AT prior to entering the induction heating unit 13, where the temperature is further increased AT preferably to between 450 and 650 F. This heating, if in excess of about 450 F., is performed in a nitrogen or other inert gas atmosphere, maintained within hood 112. As the heated strip approaches the work rolls 8 of stand 7, spray system 10 applies a lubricant to the rolls. Application of a lubricant is highly desirable in controlling the amount of friction thereby optimizing the rolling force required. In the bite of stand 9, the strip temperature is rapidly raised at least about 300 F. and preferably more than 600 F. (in approximately one hundredth second) to above the recrystallization temperature, followed by cooling AT subsequent to emergence of the strip from the roll bite. As the strip passes through the work rolls 8, it is rapidly cooled to non-oxidizing temperatures by cooling sprays 17. The rolls are simultaneously cooled by sprays 15 (which may be water, removed by wipers 16). Alternative to the use of sprays 17, the strip may be cooled by internally-cooled rolls (not shown).

The following specific example is provided to present a fuller understanding of the concepts of the invention.

Stand 1 consists of working rolls 2 of 21 inches diameter. Incoming strip (36 inches wide and 0.15 inch thick) at a temperature of 70 F., enters stand 1 at a speed of 150 ft./minute. A 1635 horsepower motor provides the power to apply a rolling force of 1710 tons and thereby achieve a single pass reduction of and a temperature rise of 145 F. The strip exiting stand 1 at a speed of 300 f.p.m., a thickness of .075 inch and a temperature of 215 F., is further heated to a temperature of 600 F., by an induction heater, supplied with 4260 kw. of power. Stand 9, equipped with -inch diameter work rolls, in a roll cluster configuration as( shown in FIG. 2) in which each backup roll is driven by a 7000 horsepower motor, provides a rolling force of 8400 tons so that the strip is given a reduction and emerges at a thickness of 0.0075 inch and a speed of 3000 f.p.m. The temperature rise of about 800 F. attendant this deformation, provides a peak temperature of almost 1400 F., and at a rate fast enough to provide annealed strip which is immediately cooled upon exiting from the rolls 8.

The contemplated process can be further enhanced by the use of automatic control means for regulating the amount of heat imparted to the strip by the induction heater 13, to thereby obtain optimum and uniform physical properties in the rolled strip. Such means are illustrated in FIG. 4. Referring to this figure, a sensor 18 is positioned on the exit side of the stand 7 to monitor the ductility (or the yield strength) of the exiting strip. This sensor may be of the magnetic type, well known to those skilled in the art. The sensor will generally have the characteristic such that a change in the physical property being monitored (for example ductility or hardness) will provide a change in signal output. In the specific embodiment illustrated, increasing ductility provides a corresponding increase in D-C voltage output. The desired and monitored physical property is preset on a potentiometer 19 connected to a source of DC power 20, and arranged so that, if the strip has the desired physical property, there will be no potential difference between the points 21 and 22. If the strip, for example, is harder than desired, 22 will be positive with respect to 21 and thereby buck the potential of voltage source 23. The potential across potentiometer 24 is thereby decreased and motor 25 (which is coupled to slider 26) rotates to increase the potential on the slider to again provide a balance, and at the same time, increase the output of heater 13. The control circuit of the heater is coupled to the motor 25, which is powered by D-C power amplifier 27. Tachometer generator 28 which is coupled to one of the Working rolls 8' of stand 7, simultaneously provides a signal to reflect changes in mill speed, which signal is likewise bucked against the potential of the voltage source 23 to control the motor 25 and likewise balance the potential and control the heater output.

Without departing from the spirit of the invention, more sophisicated control systems may be employed which measure the temperature of the strip as it enters the bite of the final standand which compute the energy to be imparted to the strip at the final stand.

I claim:

1. A method for the ultra-fast recrystallization of rolled strip, which comprises continuously subjecting the strip at a temperature T which is substantially below the recrystallization temperature T to a terminal rolling operation in which the severity of the deformation energy imparted to the strip is suflicient to raise the temperature of the strip by an increment AT whereby and thereafter reducing the temperature of said strip to below oxidizing temperatures.

2. A method in accord with claim 1, wherein prior to said terminal rolling operation the strip at temperature T is preheated in at least one additional stage.

3. A method in accord with claim 1, wherein the strip at temperature T is preheated in stages defined by where AT =temperature increase imparted to the strip by at least one prior rolling operation, and AT =temperature increase imparted by at least one heater.

4. A method as defined in claim 3 in which AT varies between and about 350 F., AT varies between about 0 and 600 F., and AT is greater than about 300 F.

5. A method as defined in claim 4 in which T is ambient temperature, AT varies between about 50 and 200 F. and is accomplished in one roll pass, and AT is at least about 200 F.

6. A method as defined in claim 5 in which AT is accomplished in one roll pass.

7. A method as defined in claim 6 in which AT is greater than 600 F.

8-. A method as defined in claim 6 in which the strip at temperature T has a thickness of from about 0.05 inch to 0.25 inch and in which the total reduction of the strip is at least 9. A method in accord with claim 7 in 'which a physical property of the recrystallized sheet is converted to a signal which is employed to control the degree of energy input to the strip, so as to maintain a predetermined physical property.

10. A method in accord with claim 7, in which a physical property of the recrystallized sheet is converted to a signal which is employed to control heater output and thereby the temperature increase (AT in relation to the speed at which the strip traverses said stages, so as to maintain a predetermined value of said physical property.

11. An apparatus for the ultraafast recrystallization of rolled strip, which includes:

(a) preheating means, for raising the temperature of the strip above its initial temperature (T (b) rolling means, which include work rolls of 30 inches to inches in diameter, driven by a torque sufficient to provide a deformation capable of raising the temperature of the strip by at least 300 F.;

(0) means for conveying said strip at a speed sutficient to prevent significant heat loss during its traverse from said means in (a) to said means in (b), and

(d) cooling means for reducing the temperature of the strip immediately after passing through said means in (b).

12. The apparatus of claim 11, wherein the torque in b) is sufficient to provide a deformation capable of raising the temperature at least 600 F.

13. The apparatus of claim 12, wherein the means of (a) include:

(i) roll means for the initial cold reduction of said strip;

(ii) heater means for raising the temperature of said strip subsequent to said means in (i), and

(iii) means adjacent said heater means for providing a non-oxidizing atmosphere about the sheet, as it travels from said heater means to said rolling means in b).

14. A mill as in claim 13, which further includes:

(a) means for sensing a physical property of said sheet and supplying a signal proportional thereto;

(b) means for supplying a signal proportional to the line speed of said sheet, and

(c) means responsive to said physical property signal and said line speed signal, for controlling the heat output of the heating means (b).

References Cited UNITED STATES PATENTS 2,767,836 10/1956 Nachtman et a1 72-41 2,767,837 10/ 1956 Nachtman et a1 72-42 3,078,191 2/1963- Maeda 148-1 1.5 3,099,176 7/1963 Hall et a1. 72-364 3,234,053 2/1966 Pryor 148-11.5 3,251,215 5/1966 Graef 72-364 3,310,389 3/1967 Doyle 148-11.5 3,392,062 7/1968 Altenpohl et al 148-11.5 3,459,599 8/1969 Grange 148-12 3,496,755 2/ 1970 Guernsey 72-364 CHARLES W. LANHAM, Primary Examiner E. M. COMBS, Assistant Examiner US. Cl. X.R. 

